<tal:block metal:define-macro="standard">

  <span tal:condition="tal_art | NULL">
    <tal:block metal:use-macro="structure/${list_art}.xhtml/${tal_art}" />
  </span>

  <span tal:condition="not:tal_art | NULL">
    <tal:block metal:use-macro="structure/${list_art}.xhtml/standard" />
  </span>

</tal:block>